The multi-billion dollar global market for semiconductor defect management is growing both in absolute terms and as a percentage of semiconductor capital equipment investment. In general, there are two factors that determine the economics of a semiconductor fabrication facility at a given utilization level, namely throughput and yield. As complex new technologies such as 300 mm wafers, copper interconnects, and reduced feature (circuit) sizes drive the margin of error in fabrication ever lower, new inspection technologies are critical to keep yields high and bottom-line economics attractive. Detection and elimination of chemical contamination and other types of defects is a constant concern for semiconductor manufacturers and equipment suppliers. Contamination can arise from use of processing chemicals, processing equipment, and poor handling techniques. Contaminants can include, for example, metals, carbon, and organic compounds. Other types of defects can result from a wide range of causes, including flaws in the semiconductor crystal, improper processing, improper handling, and defective materials. In addition, many cleaning steps are required in wafer fabrication, such as but not limited to the semiconductor industry. Each step is time consuming and requires expensive chemicals that may require special disposal procedures. Existing methods for monitoring or controlling these processes are expensive and time consuming. As a result, wafers are often cleaned for a longer period of time and using more chemicals than are required.
Defect detection and characterization systems can be divided into in-line and off-line systems. “In-line” refers to inspection and measurement that takes place inside the clean room where wafers are processed. “Off-line” refers to analysis that takes place outside of the wafer processing clean room, often in a laboratory or separate clean room that is located some distance from the manufacturing area. In addition, many of these analytical techniques are destructive, which requires either the sacrifice of a production wafer or the use of expensive “monitor” wafers for analysis. In-line inspection and measurement is crucial for rapidly identifying and correcting problems that may occur periodically in the manufacturing process. A typical semiconductor wafer can undergo over 500 individual process steps and require weeks to complete. Each semiconductor wafer can have a finished product value of up to $100,000. Because the number of steps and period of time, involved in wafer fabrication are so large, a lot of work in process can exist at any point in time. It is critical that process-related defects be found and corrected immediately before a large number (and dollar value) of wafers are affected. Such defects, regardless of the nature of the wafer, semiconductor, IC, or other device, are detrimental to performance and diminish productivity and profitability.
Many types of defects and contamination are not detectable using existing in-line tools, and these are typically detected and analyzed using expensive and time-consuming “off line” techniques (described below) such as Total Reflectance X-ray Fluorescence (TXRF), Vapor Phase Decomposition Inductively Coupled Plasma-Mass Spectrometry (VPD ICP-MS) or Secondary Ion Mass Spectrometry (SIMS). Since these techniques are used off-line (outside of the clean room used to process wafers) and usually occur hours, or even days, after the process step that has caused the contamination, their value is significantly limited.
A brief description of some well known techniques for wafer inspection and chemical contamination detection are presented in Table 1. This list is not in any sense exhaustive as there are a very large number of techniques that are used for some type of semiconductor analysis or characterization or for other surface inspection of other types of materials.
TABLE 1In-line/Analytical TechniqueDescriptionOff-lineTotalX-rays irradiate the wafer within theOff-lineReflectioncritical angle for total externalX-Rayreflectance, causing surface atoms toFluorescencefluoresce.(TXRF)AutomatedOptical images are acquired andIn-lineOpticalautomatically analyzed for detectionMicroscopyof large defects.LaserWafer surface is illuminated withIn-lineBackscatteringlaser spots and the angle and/orpolarization of reflected light isanalyzed to detect and classifyparticles.Vapor PhaseWafers “scanned” with a drop of HFOff-lineDecompositionthat is analyzed using massInductively Coupled-spectrometry.Mass Spectrometry(VPD ICP-MS)SecondaryIon beam sputters the wafer surfaceOff-lineIon Masscreating secondary ions that areSpectroscopyanalyzed in a mass spectrometer.(SIMS)
Table 2 summarizes some major advantages and disadvantages of each example technique. In general, off-line detection techniques are extremely sensitive to tiny amounts of contamination; but are slow, expensive, and complex to operate. Some have limited, or no, imaging or surface mapping capability, or are destructive in nature. In-line techniques are much faster, non-destructive, and provide defect mapping, but have limited chemical contamination detection or analysis capability.
TABLE 2Analytical TechniqueAdvantagesDisadvantagesTotal Reflection X-Very sensitiveLimited coverageRay FluorescenceSome mapping capabilityUnpatterned wafers(TXRF)NondestructiveonlyAutomated OpticalFastVery limited chemicalMicroscopyRelatively low costand particle detectionDetects a wide range ofmacro defects (>50microns)Imaging of wafer surfaceNon-contact/non-destructiveLaser BackscatteringFastOnly detectsRelatively low costparticles - noDetects very smallchemistryparticlesImaging of water surfaceNon-contact/non-destructiveVapor PhaseVery sensitiveDestructiveDecompositionAble to identify wideSlowInductively Coupled-range of contaminantsExpensiveMass SpectrometryComplex(VPD ICP-MS)Cannot imageOnly works on baresiliconSecondary Ion MassVery sensitiveExpensiveSpectroscopy (SIMS)Detects a wide range ofSlowcontaminantsDestructiveSub-surface detection
In general, existing in-line wafer inspection tools operate at production speeds and generate images of the wafer surface that are processed to identify and locate defects. However, these techniques are, as mentioned above, very limited in their ability to detect chemical contamination. Laser backscattering systems are limited to detecting particles down to sub-micron sizes, and optical microscopy systems can only detect chemical contamination that results in a visible stain or residue. Both techniques lack the ability to identify or classify the chemical composition of the particle or contamination. Off-line laboratory techniques are used to qualify the cleanliness of new processes and equipment, or to analyze defects detected by in-line equipment or as part of failure analysis.
Another system that has been investigated is the use of Contact Potential Difference imaging (CPD). CPD refers to the electrical contact between two different metals and the electrical field that develops as a result of the differences in their respective maximum electronic energy level, i.e. their respective Fermi energies. When two metals are placed in contact, the Fermi energies of each will equilibrate by the flow of electrons from the metal with the lower Fermi energy to that of the higher. “Vibrating CPD sensor” refers to the vibration of one metal relative to the other in a parallel plate capacitor system. The vibration induces changes in the capacitance with time, and therefore a signal related with the surface profile. A CPD signal can also be generated by the translation of one surface past a reference sample through the use of a non-vibrating contact potential difference (nvCPD) sensor(s). This translation makes high speed scanning possible.
However, even these nvCPD sensors can themselves present certain difficulties. At a microscopic level, the surfaces of wafers are not flat due to wafer thickness variation, materials on the surface, “bowing”, and other factors. In order to scan the wafer at a close but safe (i.e., close to the surface to promote good signal strength but far enough away to minimize any possibility of impacting the wafer surface) distance, an appropriate sensor height must be calculated and set. Thus, the height of the sensor above the wafer surface must be measured and controlled to produce repeatable results. Furthermore, height control is also necessary to minimize the sensor height to improve resolution and signal strength. However, height is difficult to control and measure, as is the appropriate height for each specific wafer.
It is possible to use one of many commercially available height sensors to control the height of the nvCPD sensor above the wafer surface. This requires the expense of an additional sensor, and the added complexity of a calibration routine to determine the position of the nvCPD sensor tip relative to measurements made by the separate height sensor.
A related problem is the difficulty in establishing a point of reference for all distance measurements, including height, related to an nvCPD scan. A reference point is needed to produce useful measurement data for image production.
In some sensor systems, such as nvCPD sensors, it is necessary to separate the sharp peak signal from the other two components of the signal (low frequency signal and induced noise signals) to locate and measure the contaminated areas of a wafer. This is challenging because the sharp peak signal behaves like noise, i.e., it consists of sharp peaks that alternate their polarity in high frequency mode. Because of this, conventional high frequency filters based on the frequency domain only do not work, as they would degrade the sharp peak signal significantly along with the noise.
In addition, an nvCPD signal is generally delayed in time, which impacts on the quality of the nvCPD signal/image. As the sampling time increases, the time delay becomes larger. The time delay may be explained by the equivalent RC circuit modeling the electrical signal path from the probe tip to the output of the A/D converter through the amplifier, the data acquisition board and the connecting lines between them. The equivalent capacitance is mixed with the capacitance between the probe and the wafer surface, the parasitic capacitance of the connecting lines, the internal capacitance of the amplifier, and other known conventional effects. The result is that minute feature signals are less detectable, and the signal magnitude and thus the signal-to-noise ratio are smaller.
A critical need therefore exists for a fast, inexpensive, and effective means of detecting, locating, and classifying relatively small quantities of chemical content and physical features on wafers. In addition, there is a need for a system which minimizes cost and complexity of the sensor control mechanisms, such as height control. Furthermore, there is a need for methods and systems that have improved signal processing.
It is therefore an object of the invention to provide an improved method and system for inspection of surfaces of any materials, such as semiconductor wafers and other electronic devices.
It is an additional object of the invention to provide an improved method and system for providing images of surface defects on a semiconductor wafer or an integrated circuit device.
It is yet another object of the invention to provide an improved method and system for identifying different classes of wafer surface defects by pattern recognition.
It is still a further object of the invention to provide an improved method and system for classifying categories of surface defects on semiconductor wafers, including without limitation surface defect states, electrostatic field variations, oxide states and chemical contamination.
It is also an additional object of the invention to provide an improved method and system for sensing electrostatic fields arising from semiconductor wafer surface defects.
It is yet another object of the invention to provide an improved method and system for detecting the presence of thin dielectric films on surfaces of semiconductor wafers and to detect film defects such as pinholes, bubbles, delaminations, or contamination under the film.
It is a further object of the invention to provide an improved method and system to sense variations in oxide states on semiconductor wafer surfaces.
It is also a further object of the invention to provide an improved method and system to classify particulate contaminants on semiconductor wafers identified initially by optical inspection systems.
It is yet a further object of the invention to provide an improved method and system for detecting variations in dopant concentration of semiconductor wafers.
It is another object of the invention to provide an improved method and system for use of an nvCPD sensor to inspect the surface quality of semiconductor wafers.
It is still another object of the invention to provide an improved method and system of nvCPD sensors in combination with other inspection systems for evaluating semiconductor wafer surface properties.
It is a further object of the invention to provide an improved method and system for producing topological images of differing contact potential characteristic of defects on a semiconductor wafer.
It is also an object of the invention to provide an improved method and system for rapidly scanning the surface of a semiconductor wafer to identify sub-microscopic, microscopic and macroscopic surface defects characterized by potential field disturbances on the wafer surface.
It is also an object of the invention to provide an improved method and system for detecting the cleanliness of a semiconductor wafer to determine if a cleaning process has eliminated all contaminants and to avoid the time and expense of cleaning wafers for longer than is necessary to remove contaminants or to perform unnecessary processing steps.
In each case described above, wafer surface can refer to the front-side (patterned side) of the wafer, back-side (unpatterned side) of the wafer, or the edge of the wafer or any surface undergoing inspection regardless of the type of material.
Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings described below.